In the practical use of the transferred plasma-arc flame spray systems for spraying of electrically conductive wires and rods, major problems confront the process. The equipment requirements of transferred plasma-arc spraying of electrically conductive wires and rods are similar to those used for plasma cutting of metal. In place of a metal to be cut, a wire, rod or strip of metal is fed into the transferred arc for melting and atomizing the metal. A large secondary flow of compressed air further atomizes the molten particles and accelerates them to high velocity for impaction against the surface to be spray-coated. FIGS. 1A and 1B illustrate a typical transfer of plasma-arc flame spray system, and the double arcing action, due to a stop-page of feed of the metal material. In FIG. 1A, the transferred plasma-arc flame spray system of the prior art takes the form of a transferred plasma-arc torch 1, comprised of three major elements. An electrode 11 is mounted coaxially within an electrically insulating piece 10 at one end of a cylindrical metal body 12, the opposite end of the body 12 is closed off by an end wall 2, provided with an axial bore forming a nozzle 30. The electrode 11 is coaxial with the nozzle passage or bore, and within an annular chamber 15. A plasma forming gas is introduced through a tube 13, and a formed passage 14 within the insulating piece 10 to chamber 15, where the plasma forming gas passes into and through nozzle 30. Concentrically surrounding the body 12 is a cup-shaped member 23, forming an annular space 31 between the cup-shaped member 23 and the cylindrical body 12. One end of the cup-shaped body 23 is closed off by end wall 23a, while its opposite end 23b is open. A tube 25 provides and feeds compressed air into the annular space 31 for discharge through the open end of the cup-shape member 23, which functions to atomize the metal fed into the plasma-arc, and accelerates those particles in the direction of the workpiece or substrate 29. The metal to be flame sprayed onto the surface of the substrate 29, is shown in the form of a wire or rod 18, which is fed into a developed arc column 17, by powered rolls 19 which rotate in the direction of the arrows to feed the wire 18 from right to left FIG. 1A. An electrical, potential difference is developed between the wire 18 which acts as one as the anode, and the cathode electrode 11 from a DC electrical source such as generator 22 via leads 20, 21 coupled respectively to the cathode electrode 11 and the anode wire 18.
In operation, the arc column 17 is positioned centrally through the nozzle 30 to strike the anode, in this case, wire 18 fed by powered rolls 19. The gas flow, (particularly where it has sufficient tangential whirling component to cause vortex flow through the nozzle 30) positions the arc column 17 essentially within the nozzle 30, well away from the nozzle wall. The compressed air which causes additional atomizing and particle acceleration of particles 27, passes from the annular space 31 through a conical discharge passage 24, exiting the annular opening 23b of member 23 as an annular, conical, high velocity air-flow or stream 26 thereby accelerating the molten particles 27 to form coating 28 on the workpiece or substrate 29.
As long as the end of wire 18 remains in line with nozzle 30, performance remains stable. High melt-off rates are possible at reasonable costs. To remain in-line with nozzle 30, the wire 18 must be fed steadily at high rates. Malfunction of the power feed system, schematically illustrated by the powered rolls 19, a kink in the wire 18, or inadvertent shut-off of the wire feed, can result in serious damage due to the phenomena of "double arcing".
FIG. 1B illustrates a double arcing action due to the stoppage of wire 18. Assuming that the mechanical feed system exemplified by powered rolls 19 has been shut-off, the stable operation of FIG. 1A will keep the arc column passing through nozzle 30 to strike and melt the receding end of wire 18. The arc column obviously must bend to the right, and within nozzle 30 it soon approaches and contacts the nozzle wall of the cup-like member 23 adjacent its open end 23b. As soon as this happens, an alternate electrical path of lower voltage becomes available to the electrical source 22. This path, is set up between the tip 11a of the cathode electrode 11, at a, the edge of cylindrical body end wall 2 at the nozzle 30 proximate to the electrode tip 11a, at b, through the cylindrical metal body 12, the cup-shaped member 23, the edge of the cup-shaped member 23 adjacent opening 23b, at c and the receding end d of metal wire 18 forming the anode electrode. The path a-b-c-d, establishes two low voltage arcs A and A' FIG. 1B, between the points a, b, and c, d respectively, with the current passing through the low resistance path of metal cylinder 12 and cup-shaped member 23 respectively. The metal at points b and c is rapidly eroded away, often leading to complete destruction of the plasma torch 1.
It is therefore a primary object of the present invention to provide an improved transferred plasma-arc flame spray system which eliminates double arcing.
It is a further object of the present invention to provide an improved transferred plasma-arc flame spray system in which both a powder and wire may be fed into the struck arc, wherein, the wire of particles strike the workpiece or substrate in a molten state, while the particles introduced in powder form are in a heat softened state at the moment of impact with the substrate.
It is a further object of the present invention to provide an improved transferred plasma-arc flame spray system which advantageously utilizes a secondary arc created between a primary metal wire fed into the arc flame for melting, and particle application to the substrate in a secondary wire fed thereto to materially increase the spray rate of molten particles applied to the substrate.